One problem in Heating, Ventilation, and Air Conditioning (HVAC) systems is high discharge pressure. Discharge pressure refers to the pressure generated on the output side of a compressor. The current solution to address high discharge pressure is a pressure switch. When the discharge pressure exceeds a pressure threshold, the switch opens shutting down the HVAC system. This event is called a “high discharge pressure trip.” The HVAC system may have multiple high discharge pressure trips until the discharge pressure is normalized.
High discharge pressure trips most commonly occur during startup operation or high outdoor ambient operation. A common cause of high discharge pressure trips during startup operation is a closed expansion device. An expansion device controls the amount of refrigerant flow through a liquid line to a heat exchanger. An expansion device may be closed if the HVAC system has been off for a long period of time and under low pressure. The expansion device often takes time to open up, creating high pressure behind it in the liquid line. The high pressure in the liquid line can cause a pressure trip. Besides startup, high outdoor ambient operation can cause high discharge pressure trips. When there is high outdoor ambient temperature the compressor must perform more work to compress the refrigerant from the low evaporator side pressure to the high condenser side pressure. The added work performed by the compressor raises the discharge pressure and may cause a high discharge pressure trip.
Certain HVAC systems use a microchannel condenser and a tandem compressor to deliver higher Integrated Energy Efficiency Ratio (IEER). An HVAC system with both a microchannel condenser and a tandem compressor are especially likely to have high discharge pressure trips. This type of HVAC system is optimized for part load conditions when an additional one to two pounds of refrigerant charge is present over the optimum refrigerant charge for full load conditions. When under full load conditions, unnecessary refrigerant is present in the HVAC system. The additional refrigerant charge can cause high discharge pressure spikes and consistently higher discharge pressure. Either condition can result in a high discharge pressure trip during high outdoor ambient operation. Accordingly, an improved configuration would be desirable to reduce the likelihood of high discharge pressure and prevent high discharge pressure trips.